1. Technical Field
The present invention relates to an endoscope beam source apparatus and an endoscope system including such apparatus.
2. Description of Related Art
Generally, to observe tissue within a body cavity, there is used an endoscope system. The endoscope system is a system which radiates, as a radiation beam, a white beam onto a portion to be observed within the body cavity, picks up a beam image due to a reflection beam from the portion to be observed using a predetermined imaging device which is capable of imaging a two-dimensional image, and displays the thus obtained two-dimensional image on a monitor screen. A technology for controlling the radiation beam of such endoscope system is disclosed in, for example, JP-2009-056248-A, JP-2007-111151-A and JP-2008-029621-A.
In JP-2009-056248-A, there is disclosed a technology for always obtaining a radiation beam having proper beam quantity and chromaticity. Specifically, there is proposed a technology in which a drive current to be applied to a beam source is caused to change in the form of a pulse and the pulse is controlled in any one of the number, width and amplitude thereof.
In JP-2007-111151-A, there is disclosed a technology for supplying a radiation beam onto a diseased part while controlling the heating of a leading end. Specifically, there is proposed a technology for controlling the lighting/lighting-out of a beam source in a pulse manner and also for adjusting the lighting time of the beam source and the amplitude (intensity) of the pulse.
According to JP-2008-029621-A, there is disclosed a technology which, when imaging a static image using a CMOS image sensor, turns on beam radiation only for a short period of time. Specifically, the charge accumulation operation is started from a state where the radiation is firstly turned off and charges are then reset at the respective pixel positions. Also, when the charges are read out from the respective pixels, the radiation is turned off. According to this technology, it is possible to prevent extra charges from being accumulated due to different timing for reading the charges from the respective pixels.
Here, a beam source apparatus for radiation used in an endoscope system is generally required to have a beam quantity dynamic range of 1:9000 or broader. Realization of such broad beam quantity dynamic range is difficult simply by controlling the amplitude of a current to be applied to the beam source.
Also, besides the control of the current amplitude, as disclosed in JP-2009-056248-A, the radiation beam quantity can also be controlled by carrying out pulse number control and pulse width control on the beam source application current. However, in any one of the pulse number control, pulse width control and pulse amplitude control, a sufficiently large beam quantity dynamic range cannot be obtained when only one of them is applied.
Also, even in the case that a beam source apparatus has multiple kinds of control functions such as the pulse number control, pulse width control and pulse amplitude control disclosed in JP-2009-056248-A, a user must adjust individually the multiple kinds of control quantities which are different in the properties (the relationship between an input value and a beam quantity variation) from each other. Therefore, an operation to adjust the radiation beam quantity to a desired value is very difficult.
Further, in the case that the beam quantity is controlled using the pulse number control, pulse width control and pulse amplitude control, unless the properties of an imaging device to be mounted on an endoscope are taken into consideration, there cannot be obtained an image of high quality.
Here, as an imaging device which can be used in the endoscope system in order to image the two-dimensional image, there are known a CCD (Charge Coupled Device) image sensor and a CMOS (Complementary Metal-Oxide Semiconductor) image sensor. Also, as known well, the signal reading systems of the CCD image and CMOS image sensors are different from each other because they are different in structure, and the two image sensors are also different in the shutter control in photographing.
For example, a CCD image sensor of an interline type includes a beam receiving portion, a vertical transfer portion, a horizontal transfer portion, an amplifier and the like. That is, since the CCD image sensor includes the vertical transfer portion capable of holding charges for all pixels, after completion of exposure, the charges of the beam receiving portion for all pixels can be transferred to the vertical transfer portion at the same timing. Therefore, the timing for starting the accumulation of the charges at the respective pixel positions of the beam receiving portion and the timing for ending the charge accumulation are simultaneous for all pixels. That is, when imaging a two-dimensional image, simply by controlling only the image sensor, the shutter can be released simultaneously for the whole of 1 frame of the two-dimensional image. This shutter control is referred to as a global shutter system.
On the other hand, in the case of an ordinary CMOS image sensor, since there is not provided a structure element such as the above-mentioned vertical transfer portion which can accumulate temporarily the charges of all pixels, it is necessary to read charges sequentially line by line from the respective pixel positions of a beam receiving portion of a two-dimensional arrangement constituted of N lines and M rows. That is, as in the screen scan of a TV set, while switching scan lines sequentially, charges are read out for every line. Therefore, the timing for starting the accumulation of the charges at the respective pixel positions of the beam receiving portion and the timing for ending the charge accumulation vary slightly in every line. In other words, when imaging a two-dimensional image, simply by controlling only the image sensor, timing for releasing the shutter varies in every line of the two-dimensional image, whereby the shutter cannot be released simultaneously for the whole of 1 frame. This shutter control is referred to as a rolling shutter system.
Therefore, in the case of an endoscope system employing an ordinary CMOS image sensor, the timings in the charge accumulation period (the time during which the shutter is substantially opened) at the respective positions of the beam receiving portion is different every scan line. Therefore, in the case that the on start timing of the beam source is adjusted in order to control the radiation beam, the radiation beam quantity varies every scan line of the two-dimensional image, thereby causing the luminance of the image to vary.
In the case that only the amplitude (beam emission intensity) of a current to be supplied to the beam source is controlled, since the radiation beam quantity is not influenced by the difference of the timings for signal reading or the like, even in an endoscope system employing an ordinary CMOS image sensor, there is no possibility that the luminance can vary every scan line.
On the other hand, in an endoscope system employing a CCD image sensor, since the timings for signal reading and the like is not different every scan line, the on start timings of the beam source can also be adjusted in order to control a beam for radiation. Also, in an endoscope system employing a CCD image sensor, since there exists the time during which the shutter is closed simultaneously for all pixels, during this time, unnecessary radiation can be turned off, which is useful in controlling heat generation. However, in an endoscope system employing an ordinary CMOS image sensor, since the time during which the shutter is closed varies every scan line, radiation cannot be turned off during a specific period.